Solution Review: Convert String Into an Integer

# include <stdio.h>
int stringToInt (char *str);
int main( )
{
	char str[ ] = {'1', '2', '3', '\0'};
    printf ("The String is: %s\n", str);
	printf ( "The number is: %d\n", stringToInt(str) ) ;
	return 0 ;
}
int stringToInt (char *str){
    // Initialize variables
	int num = 0, i ; 
	// Traverse string
	for ( i = 0 ; str [  i  ] != '\0' ; i++  )
	{
		// If string contains digit convert it into integer
		if ( str[ i ] >= 48 && str[ i ] <= 57 )
			num = num * 10 + ( str[ i ] - 48 ) ;
		else
		{
			// If string does not contain digit return -1 
			printf ( "Not a valid string\n" ) ;
			return -1 ;
		}
	}
	return num;
}